OK... so I've actually gotten myself caught up and enough time to try to research this. Now for the good, the bad, and the ugly.
That is a 21 year old bike. And that it is still running is absolutely commendable for a 1000cc sportbike.
Now... that's a 21 year old bike. Parts and support are going to be difficult. I've looked at Ractech website (disclaimer: I am not employed by, or compensated by Racetech in any way. I am not rich, and their products offer me good bang for buck). The don't have anything listed for the 87 FZR, but they do say call them, and everytime I've called, they can find the parts to rebuild. With the age of that shock I'd say it will need a spring and a recharge. Now depending on how much labor runs to do that, you may want to look into some of their other services and parts, as they typically don't increase labor that much.
Aside from that, you can look on eBay for Fox Racing, Penske, or Ohlins shocks from that era, and bear in mind those parts will likely need overhaul as well. Advantage: Ractech working over your shock.
When it comes to the front: If the forks have been serviced every two years or so, the seals are good, and you can get reasonable sag settings with them; that's probably as good as it gets. But, if you need any work at all, again, go with Racetech. Up front they have springs of course, and valve emulators. These little setups are worth their weight in gold, because they can build the valving to your riding preferences. Bearing in mind with the 87, the only damping control we get is with built in valving (emulators) and changing the oil weights.
You can get the spring specs for the front on Racetech's Site
. But nothing online for the back.
The reason I have gone into detail on the front and rear of your suspension is simple. Everything works together as a system. If you manage to get someone to setup the rear shock properly, you will likely find that your front is out of whack. I'd recommend that you look at how to get all of it done.